I think it has something to do with the theme because I see it creates a custom post type “portfolio”, try activating the plugin then open the permalink settings page (this will flush the permalink structure), see if this works.

Any ideas for next steps in getting this to work, Hesham?
No other plugin seems to have an issue so I’m wondering whether your content type needs some looking into, in relation to permalink structure compliance or something?
This is a widely sold theme by a solid developer too.

I’ve sent you a couple of emails along with details in obtaining in the theme files as well.
Just wondering whether you got those emails and/or determined why a pretty bare-bones site might experience these permalink issues with just this plugin?